1. Field of the Invention
This invention relates to a Bragg grating optical waveguide laser source comprising two or more Bragg gratings in a rare earth doped waveguide and an optical pump source coupled to said doped fibre.
2. Description of the Related Art
In wavelength division multiplexed (WDM) telecommunication systems as well as WDM sensing systems there is a need for wavelength selectable and/or tunable narrowband laser sources. In future dense WDM (DWDM) telecommunication systems and networks wavelength selectable lasers seem very attractive both as back-up lasers in multiwavelength laser transmitter modules and as wavelength switchable sources for wavelength routing/switching in optical networks. The first application typically requires very wide wavelength tuning such that one tunable laser can be a back-up for many fixed wavelength sources, but does normally not require very fast tuning. The latter application requires fast wavelength switching, typically  less than 1 ms. Future DWDM systems will have a large number of wavelength channels with very narrow channel spacing (50 GHz or less). This requires very tight laser wavelength control combined with robust single mode laser operation with narrow linewidth (less than a few MHz) and high side-mode suppression ratio (SMSR). In addition tunable lasers should ideally provide wide tuning ( greater than 40 nm) combined with high power ( greater than 20 mW). There are several tunable multi-section semiconductor lasers under development which are aiming at the applications mentioned above [1]. However, these multi section lasers are complex, with up to 4 different currents required to control both wavelength and power and ensure mode-hop free single mode operation, and are difficult to make with both high power and wide tuning. A wavelength selectable, mode-hop free single mode laser with high SMSR can be realised by combine several semiconductor DFB laser cavities with different wavelengths on a single chip in a linear gain-coupled [2] or a parallel index-coupled [3] DFB array, with separate drive currents such that only one laser is pumped and oscillating at a time. All laser cavities can be temperature tuned simultaneously. Exact wavelength control of such lasers require wavelength lockers.
DFB fibre lasers are another attractive alternative for wavelength accurate, mode-hop free tunable single mode laser source, which seem able to satisfy the requirements described above. A DFB fibre laser typically consist of a phase-shifted fibre Bragg grating (FBG) written in a rare earth doped (eg. erbium doped) optical fibre, which when pumped by a semiconductor pump laser provides lasing at wavelength determined by the grating [4]. With an optical phase-shift of "PHgr"=Π/2 at the centre of the grating the round-trip phase condition of n*2Π (n is an integer) of the laser cavity is satisfied at the centre (Bragg) wavelength of the grating, providing the lowest lasing threshold. Detuning the phase-shift away from the optimum value of Π/2 will increase the threshold and possibly prevent laser operation [4]. The wavelength of a DFB fibre laser can be set with great accuracy and repeatability in the writing of the grating, and with proper annealing the wavelength remains stable over time. DFB fibre lasers will inherently operate at two polarisations, but single-polarisation operation can be obtained for example by making a grating with differential strength for the two polarisation states. The temperature sensitivity of the laser wavelength is one order of magnitude lower than of a semiconductor DFB laser, relaxing the requirements on temperature stabilisation. The wavelength of a DFB fibre laser can be tuned continuously without mode-hopping through uniform strain/compression and/or temperature tuning of the whole laser cavity [5], [6]. Such mechanical or thermal tuning is relatively slow (typically  greater than 0.1 s). The tuning range with temperature is typically  less than 1-2 nm due to the low temperature sensitivity. Strain/compression tuning allows tuning over typically 5-15 nm, mainly limited by the mechanical reliability of the fibre. Good control of strain/compression and temperature can provide highly accurate wavelength control without the need for closed loop control and wavelength lockers. Note that DFB fibre laser can not be directly modulated at telecommunication speeds due to the slow gain medium, and require external modulators.
A wavelength selectable fibre laser has been demonstrated using a laser cavity comprising a widely tunable filter based reflector combined with a series of FBG reflectors [7]. A wavelength selectable fibre laser system can also be made by pumping several lasers having different wavelengths with one optical pump source, where lasing at one wavelength at a time can be realised by having an optical switch between the pump source and the fibre lasers [8].
Continuously wavelength swept mode-hop free and narrow linewidth lasers are very attractive for high resolution, fast spectral characterisation of wavelength dependent optical components such as FBG filters.
The objective with the present disclosure is to provide a single longitudinal mode fibre laser which can preform rapid wavelength switching between two or more wavelengths, with switching times  less than 1 ms. A second objective is to provide a robustly, mode-hop free single longitudinal mode fibre laser which can be tuned to any wavelength within a wide wavelength band, requiring only two tuning control parameters. A third objective is to provide a narrow linewidth ( less than 1 MHz) laser which can be continuously wavelength swept to measure the spectral characteristics of wavelength dependent optical components at any wavelength across a wide wavelength range. A forth objective is to provide a laser source which can provide lasing at more than one wavelength with control of the number of lasing wavelengths and where these wavelengths can be continuously tuned.
The objectives can be met by having several fully/partly spatially overlapping DFB fibre lasers in the same rare-earth doped fibre with different grating pitch and different phase-shift, pumped by one or more optical pump sources, which are typically a semiconductor laser. The DFB laser gratings can be written overlaid (one by one), or as a complex sampled grating with an index profile equal to a sum of the individual laser gratings. All the lasers have the same phase-shift position, but with different induced central phase-shifts xcfx860i (i=1,2, . . . n, where n is the number of lasers) such that only one laser at the time has an optimum total phase-shift xcfx86=Π/2xc2x1xcex4xcfx86 where 2xcex4xcfx86 is the range of phase-shift providing acceptable lasing conditions. The individual lasers can be turned on by changing the phase-shift of that particular laser by a controlled amount xcfx86. The change in phase-shift can be introduced by local heaters or PZT stretchers, or by locally induced changes in refractive index (using for example an electro-optically active poled fibre). In a preferred embodiment all overlaid lasers have the same phase-shift position in the fibre, but have different inherent phase-shifts, such that for a given induced phase-shift only one laser is oscillating at a time, having a total phase-shift of "PHgr"=Π/2xc2x1xcex4xcfx86, while the other lasers have phase-shifts outside this range, and hence threshold levels too high to provide lasing. Typically, but not necessarily, the difference in peak reflection wavelength of the Bragg grating, hereafter called Bragg wavelength (grating pitch) between the individual DFB fibre lasers is so large that the reflection spectrum associated with the individual lasers have no spectral overlap. Changing the phase-shift will then allow switching between laser wavelengths separated by typically the separation between the Bragg wavelengths of the individual laser gratings. The speed of this switching will be limited i) by how fast the correct phase-shift can be introduced in the core of the fibre, and ii) by the laser cavity lifetime, which is typically 0.1-1 xcexcs. By making the laser wavelengths matching selected ITU wavelength, fast switching between ITU wavelength channels can be realised.
In a second embodiment the phase-shifts for the individual DFB fibre lasers have different positions with individual phase-control (with eg. heaters or stretchers) such that each laser can be turned on/off individually. This can provide lasing at one or more laser wavelengths simultaneously.
The objectives can also be met by having non-overlapping or partially overlapping DFB fibre lasers with different grating pitch, either written in the same fibre or in separate fibres, where the phase-shifts of each laser can be tuned/switched independently to turn the lasers on/off independently.
Each laser wavelength can be tuned continuously by straining/compressing (or heating/cooling) the laser, enabling tuning over typically xcex94xcex=5-15 nm. Combining the tuning range of the individual lasers with wavelength spacing xcex94xcex allows tuning over m*xcex94xcex, where m is the number of DFB fibre lasers, where typically only one laser is lasing at a time. An important advantage of the approach is that all lasers can be strained/compressed simultaneously using the same actuator, reducing the volume and complexity of the combined laser array. To cover a widest possible tuning range one can either have a large number (m) of lasers, or make the lasers with a large wavelength spacing xcex94xcex, which still will allow mechanical tuning over the wavelength range xcex94xcex.
The laser wavelength can be tuned to any wavelength within the total tuning range by controlling only two parameters, the strain/compression of the laser cavity and the induced phase-shift. This will require the temperature of the laser to be measured such that the induced strain/compression can compensate for temperature induced changes in wavelength. Alternatively passive, mechanical temperature compensation can be used to eliminate temperature induced wavelength shifts.
Alternatively the laser(s) can be operated in a wavelength sweep mode for spectral characterisation of optical components using a detector to measure the reflected or transmitted spectral power of the optical component. In the latter case normally only the wavelength range corresponding to one of the lasers can be covered at a time with continues wavelength tuning. The combined wavelength range m*xcex94xcex can be covered by doing the first sweep with only the first laser on, the second sweep with only the second laser on, and so on. Several wavelength ranges can be covered simultaneously be operating several lasers in on-mode simultaneously and separate the wavelength scans with a wavelength demultiplexer having separate detectors at the output.
The laser wavelength can be controlled prior to the onset of the individual lasers by either measuring the strain/compression with a capacitance positioning sensor and the temperature with a temperature sensor, or alternatively by measuring the combined effect of strain/compression and temperature with an FBG sensor imprinted in the strained/compressed fibre section [Norwegian patent application 1999.5485]. The phase-shift of each laser can be set to the correct magnitude by introducing a pre-determined local increase in refractive index and/or strain at the centre of the grating, or alternatively by tuning the phase-shift to maximise the output power using a feedback loop.